BACKGROUND: Twinning is associated with heightened risk of cerebral palsy (CP) and is increasing in the United States and elsewhere. METHODS: Twins with moderate or severe congenital CP were identified in a cohort of 155,572 children born 1983 through 1985 in four northern California counties and surviving to 3 years. The prevalence of CP in twins and factors associated with increase in risk were examined. RESULTS: Among 2985 twins, 20 children in 18 pairs had CP. The prevalence of CP was 6.7 per thousand 3-year-old twin children (95% confidence interval [CI], 4.2 to 11), 12 per thousand twin pregnancies (95% CI, 7.2 to 19), and 1.1 per thousand singletons (95% CI, 0.97 to 1.3). Ten percent of all CP was in twins; 22% of CP in infants of less than 1500 g birth weight occurred in twins. Twins were over-represented among very low birth weight infants but their risk of CP was comparable with that of very low birth weight singletons. Twins born weighing 2500 g and more had a CP risk 3.6 times that of singletons of similar weight. In children who survived fetal death of a co-twin, CP was 108 times more prevalent (95% CI, 42 to 273) than in singletons and 13 times more prevalent (95% CI, 4.5 to 37) than in twins whose co-twin was born alive. The CP rate in unlike-sex pairs was 13 per thousand (95% CI, 4.8 to 32), not significantly different from 11 per thousand (95% CI, 5.7 to 19) for like-sex pairs. CONCLUSION: Twin pregnancies produced a child with CP 12 times more often than singleton pregnancies. The heightened risk was largely related to the tendency of twins to be low in birth weight and to a greater risk of CP in twins of normal birth weight compared with singletons of similar weight. Twins of unlike-sex pairs, necessarily dizygotic, were not at lower risk than like-sex pairs. The current increase in multiple births is likely to contribute more children with CP.